The manufacture of micromechanical electrical system (MEMS) such as pressure sensors and other devices incorporating a membrane poses serious challenges because of the sensitivity of the devices. Typically, the devices which are made of silicon, (polysilicon or silicon-germanium) must exhibit low stress values or predetermined stress values along with low or specific stress gradient properties. Stress reduction is accordingly typically achieved during a stress/stress-gradient relief step during a high temperature annealing process.
MEMS devices however, can be very complicated devices with a number of mechanical parts that are integrated with other permanent and/or temporary (sacrificial) materials. The integrated parts may exhibit detrimental interactions due to the thermal budget. Thus, subsequent annealing steps could affect the layers previously deposited/annealed and therefore modify the film stress and stress gradient values of the devices. Thus, the timing and manner in which stress relief is accomplished must be carefully planned. This adds complexity and costs to the manufacturing process.
Various attempts have been made to control stress in the prior art. Some of those attempts include development of specialized films. While effective at reducing stress, these films suffer various shortcomings such as lack of conductivity, roughness, and irregular electrical properties. Other approaches include the use of doping or specific atmosphere control while depositing films. These approaches affect the chemical composition of the films.
What is needed, therefore, is a simple and effective approach to modification of stress characteristics within a membrane. A further need exists for an approach to modification of stress characteristics within a membrane that does not alter the chemical composition membrane.